In the construction industry drywall is almost universally used as the wall covering for interior walls and ceilings of buildings, particularly for residential housing. Drywall, also known as gypsum board, includes a central layer of hard gypsum covered on each side by a layer of heavy paper. In most types of drywall the longitudinal edges are somewhat rounded and the thickness of the sheet adjacent each edge is slightly reduced for about 5 cm from the edge. The thickness reduces gradually and the reduction takes place only on the interior surface of the sheet, that being the surface facing the interior of the room. When two sheets are butted edge to edge the adjacent reduced areas form a "groove" or joint which can be filled with a gypsum-based joint compound which, after it has set, can be sanded smooth to eliminate any effect of the joint. When the wall is painted, or covered with wallpaper or other covering, the wall will have the appearance of being continuous rather than being made up of a series of drywall panels, the width thereof being typically four feet.
Typically a drywall joint is filled in a number of steps, the first entailing the application of a layer of compound over the length of the joint. Then a strip of drywall tape, usually of paper, is applied to and embedded in the wet compound. The tape gives body to the joint and, when the edges of the sheet are not butting tight against each other, it bridges the gap between the edges and helps reduce the amount of compound required. Thereafter additional layers of compound are applied to the joint, over the tape, and they are of increasing width, being feathered outwardly to merge smoothly with the surfaces of the drywall sheets. Most of the compound, and the tape, will be located within the groove defined by the thinner edge regions of the drywall sheets, so that there is little or no outward bulging of the compound.
As an alternative to the paper tape, it is possible to use an open mesh tape which provides a screen-like texture to which the compound can readily adhere. Furthermore, it is possible to use tape having a pressure sensitive adhesive coating on one side so that the tape can be adhered directly to the mating surfaces of the drywall, thereby eliminating the initial application of joint compound. With the adhesive-backed tape the jointing of drywall sheets used on a ceiling is particularly enhanced since the wet compound has a good backing to which it can adhere and consequently there will be less dropping of compound from the ceiling to the floor.
When applying tape, whether plain paper, mesh or adhesive backed, it is necessary to take the tape from a roll and to apply it to a joint which may be as short as a few inches or as long as thirty or forty feet (if a long joint, say extending up a wall, across a ceiling and down another wall, has been created). The application of the tape can be very messy and at best it will be awkard over long distances. Furthermore, it is difficult to apply uniform pressure to the tape so that it is uniformly embedded in the joint compound, without air bubbles in the case of plain paper tape, and so that it is not wrinkled or straying excessively from the joint line. There is therefore a need for an applicator that the workman can easily manipulate for both ceiling and wall work and which has provisions for easy tape replacement and cutting. When used for ceiling joints the applicator should be designed so that the workman does not have to mount a stepladder to get close to the ceiling.
Another area in which problems exist is in the corners, both vertical and horizontal. First of all it is unlikely that sheets of drywall will meet at their edges in corners and hence the preformed edge groove is not available to receive the tape. Thus it is even more important that the tape lie flat against the surfaces of the drywall sheets. Secondly, when manually applying tape to corners one must first crease the tape longitudinally thereof to place it in a "V" shape so that it can be readily and reasonably accurately placed in the corner. Most tapes have a preformed crease line therein extending along the centerline thereof to facilitate the creasing or folding step. Lastly, an applicator tool, many of which have been tried, and have failed, commercially, which works well on wall joints between coplanar wall joints, is ineffective in applying drywall tape to a corner, whether the corner be vertical, as formed between abutting wall sheets, or horizontal, as formed between abutting wall and ceiling sheets.